This invention relates generally to power supply systems and more particularly to power supplies for use in such systems which are capable of operating at reduced output voltage with controlled current.
Controlled inverters are frequently employed as the voltage generating source for power supplies. These inverters have a maximum current limit which, if exceeded, can damage the inverter. These inverters have primary and secondary circuits which are transformer coupled. The primary circuit includes a switching means which responds to pulsating drive signals of varying pulse width and duration to control the current in the primary circuit and thus the power coupled to the secondary circuit. The output voltage of the power supply is taken from a rectified filter connected to the transformer secondary winding.
It is well known in the art that excessive loading of a power supply causes its output voltage to decrease and the inverter current to decrease. If the magnitude of the current is not maintained at some reasonable value the inverter and possibly other power supply circuits can be destroyed.
It is customary to connect the outputs of a plurality of power supplies in parallel to provide a common voltage to an electrical load. This is normally done for either or both of two reasons, (1) to increase system reliability (i.e. each power supply serves as a back-up for the other(s), or (2) to handle the power (current) requirements imposed by the load. Frequently the load will draw more current than one power supply can handle. Thus, paralleling of power supplies increases the power supply system current capabilities.
It is also well known in the art that unbalanced loading will sometimes cause one power supply in a paralleled system to go into a current limit condition (overloaded) while the other supply(s) does not. In a paralleled system, when this condition occurs, it is desirable to allow the other supply(s) to pick up the load while still allowing the overloaded supply to continue operating near its maximum current even if it operates at a reduced voltage.
In all known prior art systems inverter current is monitored by circuitry connected to the secondary circuit of the inverter. These currents are extremely high (e.g. 50 - 100 amperes) and require expensive circuits to monitor these currents. Further, when the power supply goes into a current limit condition an output from the monitoring circuitry generates an output signal to shut the power supply down, thus removing it from the system and throwing the entire load on the other supply(s).
In view of the preceeding, it is desirable to provide an improved power supply for use in a paralleled power supply system capable of continuous operation in the system while in a current limit condition.